Magnetic flowmeters (or magmeters) measure flow by Faraday induction, an electromagnetic effect. The magnetic flowmeter typically includes a flowtube and a transmitter. The flowtube includes a pipe, a field coil (which may include multiple coils) mounted on the pipe, and electrodes that extend through the pipe. The transmitter energizes the field coil to generate a magnetic field across a pipe section, and the magnetic field induces an electromotive force (EMF) across the process flow. The resulting potential difference (or voltage) is sensed using a pair of electrodes that extend through the pipe section and into contact with the process flow, or via capacitive coupling. The flow velocity is proportional to the induced EMF, and the volumetric flow rate is proportional to the flow velocity and flow cross-sectional area. The transmitter receives the sensed voltage from the electrodes and produces a signal representing measured flow.
In general, electromagnetic flow measurement techniques are applicable to water-based fluids, ionic solutions and other conducting flows. Specific uses include water treatment facilities, high-purity pharmaceutical manufacturing, hygienic food and beverage production, and chemical processing, including hazardous and corrosive process flows. Magnetic flowmeters are also employed in the hydrocarbon fuel industry, including hydraulic fracturing techniques utilizing abrasive and corrosive slurries, and in other hydrocarbon extraction and processing methods.
Magnetic flowmeters provide fast, accurate flow measurements in applications where differential pressure-based techniques are disfavored because of the associated permanent pressure loss (for example, across an orifice plate or Venturi tube). Magnetic flowmeters can also be used when it is difficult or impractical to introduce a mechanical element into the process flow, such as a turbine rotor, vortex-shedding element or Pitot tube.
Some magnetic flowmeters use a field coil driven directly by AC line power. Another type of magnetic flowmeter, commonly referred to as a pulsed DC magnetic flowmeter, excites or powers the field coil periodically with a low frequency square wave. Pulsed DC magnetic flowmeters utilize a magnetic field which changes direction at a frequency determined by the square wave excitation.
In magnetic flowmeters, the coil current and the number of windings of the field coil determine strength of the magnetic field perpendicular to the conductive process fluid flowing through the flowtube. The flow rate of the process fluid cutting across this magnetic field produces a small potential on the electrodes exposed to the process fluid. The signal produced on the electrodes is directly (linearly) proportional to flow rate for a given number of windings (turns) and the given coil current in the windings.
At a given flow rate, the typical noise spectrum produced by fluid flow through a magnetic flowmeter exhibits a magnitude proportional to 1/frequency (often referred to as “1/f noise” or “pink noise”). The noise level also increases with higher flow velocities. It is often advantageous, therefore, for magnetic flowmeters to be able to operate at a higher excitation frequency in order to improve signal-to-noise ratio. It is also desirable to be able to operate at a higher coil current in order to improve signal-to-noise ratio, because the signal level produced at the electrodes of the magnetic flowmeter is directly proportional to amp turns of the field coil (i.e., the product of the coil current in amps times the number of windings or turns of the field coil).
In pulsed DC magnetic flowmeters, the magnetic field changes direction at the excitation frequency. When the magnetic field changes direction, a spike in the voltage sensed by the electrodes is created by the rapidly changing magnetic field. In order to make a flow measurement, the electrode voltage measurement circuitry of the flowmeter must wait until the electrode voltage spike settles before making a measurement. Another source of potential error is that if the current in the coils has not stabilized, or if changes in the magnetic field lag behind the current and are still slowly changing, the resulting voltage coupled into the electrode signal will appear as an erroneous flow reading. At some maximum frequency, the coil drive circuit is unable to get the coil current or the magnetic flux density to settle, and consequently the changing coil current induces changes in the electrode voltage which are not related to flow. Operating above that maximum excitation frequency will cause accuracy and repeatability of the flow measurement to suffer. An unsettled current drive reduces the zero accuracy of the flow measurement.
The rate at which the coil current settles is different for flowtubes of different diameters and constructions. A smaller diameter flowtube can be operated at higher excitation frequency than a larger diameter flowtube, because coil current will settle more rapidly with a smaller diameter flowtube. Additionally, the magnetic and electrical properties of some materials used for flowtube construction may cause the magnetic flux density to lag behind the current in the coils. This is especially important for coils operating at higher frequencies.
The primary influences on the drive current waveform are the inductance and resistance of the field coil together with the leads between the coil drive circuitry and the flowtube. Magnetic flowtubes have a wide variety of different inductance and resistance values, depending on flow diameter and vendor. Different installations of the same flowtube may have sensor resistance that varies by more than a factor of two, depending upon the amount of resistance in the wires used to connect the transmitter to the flowtube. Even within a single vendor's product line, there may not be a direct relationship between inductance of the field coil and the flow diameter of the flowtube. New flowtubes, as well as design changes in existing models of flowtubes can result in different coil inductance and resistance values that were not available when a particular transmitter was manufactured.
As a result, determination of an appropriate operating point for a particular flowtube and transmitter combination can be difficult. An optimal setpoint for operating current and frequency generally cannot be predetermined.